1. Field of the Invention
The present invention relates to an active matrix type liquid crystal display device, and a matrix substrate used for this liquid crystal display device.
2. Description of the Related Art
It is known that an active matrix type liquid crystal display device is formed by facing two glass substrates against each other and fixing these, with liquid crystal being sealed in the gap therebetween. A transparent common electrode is formed on one glass substrate, and a great many transparent pixel electrodes are formed in matrix fashion on the other glass substrate, with circuitry also being formed for applying voltage individually to each of the electrodes.
FIG. 22 illustrates a common configuration of such an active matrix type liquid crystal display device, and more specifically is a plan view of the side of the device on which the pixel electrodes have been formed.
This active matrix type liquid crystal display device has a pixel matrix PX (i, j) of m rows and n columns (wherein i=1 to m and j=1 to n), a portion thereof being shown in FIG. 22. In the Figure, rectangles arrayed vertically and horizontally are represented by broken lines, each representing a pixel.
As shown in the Figure, the pixels are arrayed horizontally (in the column direction) and vertically (in the row direction), with an n number of data lines Dj (j=1 to n) corresponding with each column of these pixels being formed, and further, an m number of gate lines Gi (i=1 to m) corresponding with each column of these pixels are formed. Now, each of the data lines Dj (j=1 to n) are lines for supplying signal voltage to each pixel PX (i, j) (i=1 to m, j=1 to n). Also, each of the gate lines Gi (i=1 to m) are lines for supplying gate voltage to each pixel PX (i, j) (i=1 to m, j=1 to n), for writing the signal voltage to the pixels.
In addition to the above pixel electrode, each pixel PX (i, j) has a TFT (Thin-Film Transistor) 1. This TFT 1 has the source electrode thereof connected to the data line Dj, the gate electrode connected to the gate line Gi, and the drain electrode connected to the pixel electrode. Now, liquid crystal is sandwiched between the pixel electrode and the above-mentioned common electrode. The capacity 2 shown in FIG. 22 represents the liquid crystal capacity sandwiched between the pixel electrode and the common electrode. The TFT 1 serves as a switching device for switching between whether or not to write to the pixel, i.e., whether or not to apply the signal voltage supplied via the data line Dj to this liquid crystal capacity 2.
Next, description will be made regarding the operation of this active matrix type liquid crystal display device. With this active matrix type liquid crystal display device, an m number of gate lines Gi (i=1 to m) are sequentially scanned, and one screen image is displayed for each field cycle. Now, there are two types of methods for scanning gate lines, i.e., interlaced and non-interlaced. FIG. 23 is an example wherein m=480, and illustrates the scanning order of date lines in the two methods.
With the non-interlaced method, one field cycle is used to sequentially apply gate voltage to the 480 gate lines G1 through G480 at a certain time each, following which the same operation is performed each time the field cycle is renewed, as shown in FIG. 23. Such applying of gate voltage to the gates is performed by an unshown gate driver.
In each field cycle, gate voltage is applied to each gate line G1 though G480 once. Now, let us say that gate voltage has been applied to a gate line Gi. The gate voltage is applied to the gate of each TFT 1 of the n number of pixels PX (i, j) (j=1 to n) comprising the No. i row of the pixel matrix, so these TFTs 1 are conducting. Also, during the period wherein gate voltage is being applied to this gate line Gi, n pixels worth of signal voltage is output from unshown data drivers to each of an n number of data lines Dj (j=1 to n). The n pixels worth of signal voltage is applied to each of the liquid crystal capacities 2 of each of the pixels PX (i, j) (j=1 to n) by means of passing though the above conducting TFTs 1. Consequently, one horizontal scanning line of the image is displayed by the n number of pixels PX (i, j) (j=1 to n). Such applying of date voltage and signal voltage is performed for the first row of the pixel matrix to the 480th thereof, thereby displaying the image for one screen.
Conversely, with the interlaced method, as shown to the right in FIG. 23, in a field sequence, gate voltage is applied to the odd-numbered gate lines G1, G3, G5, and so forth through G 479, for example, following which in the next field sequence, gate voltage is applied to the even-numbered gate lines G2, G4, G6, and so forth through G 480, i.e., different gate lines are scanned in the field cycles, so the operation of displaying the image for one screen with two field cycles is repeated.
With the interlaced method, each gate line Gi is applied with date voltage only once every two field cycles, and thus is advantageous in that electrical power consumption can be conserved.
Now, the above-described known active matrix type liquid crystal display device has data lines for each column comprising the pixel matrix, so in the event that there is a great number of pixels per row, a great many number of data drivers need to be used, accordingly. However, data drivers are relatively expensive parts, and using a great number of these would make the entire device expensive. For example, a VGA liquid display panel with 1920 pixels in the column direction and 480 pixels in the row direction has 1920 data lines and 480 gate lines. In the event that data drivers and gate drivers having 240 output terminals were used to construct this liquid crystal panel according to the above-described known technique, there is the need to provide eight data drivers in the column direction and two gate drivers in the row direction. Using eight data drivers would make the liquid crystal panel expensive.
Also, the above-described known technique has been problematic in that it has been difficult to construct a liquid crystal display panel with a small display area. That is, a great number of terminals for supplying signal voltage to the above data lines are provided at the data line terminal portion which is the edge portion of the liquid crystal display panel, and this data line terminal portion needs to be reduced in size for a liquid crystal display panel with a small display area. In order for this data line terminal portion to be reduced in size, the pitch of the terminals corresponding to the above data lines must be narrowed, but the liquid crystal panel according to the known technique uses a great number of data lines, so the requirement to narrow this pitch is severely demanding. Accordingly, manufacturing of the data wiring terminal portion is more difficult, which in turn causes problems such as decrease in yield.
The present invention has been made in light of the above-described problems, and accordingly, it is an object of the present invention to provide an active matrix type liquid crystal display device which can drive each pixel with a fewer number of data lines as compared to the known art, and to provide a substrate used for the same.
The active matrix type liquid crystal display device substrate according to the present invention is arranged such that a plurality of data lines and a plurality of gate lines are provided on a substrate in a matrix form, and that on either side of each of the data lines are provided TFTs and pixel electrodes connecting to the TFTs, corresponding with each of the plurality of gate lines, wherein the plurality of data lines are provided so as to control the pixel electrodes on either side of the data lines, by signals from the corresponding one of the two gate lines on either side of each of the pixel electrodes.
With active matrix type liquid crystal display device substrate according to the present invention, one data line supplies signal voltage to the pixel electrodes positioned on either side thereof. Also, signal voltage is written to half of the pixel electrodes arrayed along the gate line by applying gate voltage to one of the two gate lines positioned on either side of pixel electrodes on either side of the data line, and signal voltage is written to the other half of the pixels by applying gate voltage to the other gate line. Thus, with the substrate according to the present invention, the number of data lines is reduced to half of that of known arrangements, thereby facilitating reduction of the expensive data drivers to half.
Also, it is preferable that the gate electrodes forming the TFTs are comprised of the gate lines themselves, and the drain electrodes which comprise the TFTs and are electrically connected to the pixel electrodes traverse the gate electrodes.
With such an arrangement, even in the event that there is shifting of the photo mask between the gate electrode forming step and drain electrode forming step in the process of manufacturing the active matrix type liquid crystal display device substrate, the parasitic capacity Cgd between the gate and drain of the two TFTS sandwiched between neighboring data lines is equal as in a normal case, and the field-through voltage xcex94Vp is also equal, thereby facilitating prevention of flickering and irregularities in brightness.
Also, the active matrix type liquid crystal display device substrate according to the present invention may be arranged such that storing capacity is provided corresponding to each of the pixel electrodes, and storing capacity lines are provided parallel to the data lines, between neighboring pixel electrodes between neighboring data lines, and that one of the electrodes of the storing capacity is connected to a pixel electrode corresponding thereto, and the other electrode is connected to the storing capacity line.
According to the present invention, storing capacity is connected to each pixel electrode, so the capability for each pixel to hold signal voltage can be improved. Also, two pixels worth of writing current flow to each storing capacity line from each storing capacity on either side. Accordingly, outputting signal voltage to each data line so that signal voltage of reverse polarity is applied to neighboring data lines causes cancellation of writing current flowing through each of the storing capacity lines, thereby preventing insufficient writing from occurring.
The active matrix type liquid crystal display device according to the present invention comprises a pair of opposedly positioned substrates whereby liquid crystal is held therebetween, wherein one of the substrates is the above-described substrate.
Also, the active matrix type liquid crystal display device according to the present invention comprises scanning means wherein an action of sequentially supplying gate voltage to one of the two gate lines provided on either side of the pixel, and an action of sequentially supplying gate voltage to the other of the two gate lines provided on either side of the pixel, are alternately performed each time a field cycle switches.
According to the present invention, signal voltage is written to all pixels in the pixel matrix over a period of two field cycles. Accordingly, consumption of electricity at the time of writing signal voltage can be reduced.
Also, the active matrix type liquid crystal display device according to the present invention comprises: gate drivers for sequentially outputting gate voltage from output terminals in each field cycle; and demultiplexers wherein an action of sequentially supplying gate voltage to one of the two gate lines provided on either side of the pixel, and an action of sequentially supplying gate voltage to the other of the two gate lines provided on either side of the pixel, are alternately performed each time a field cycle switches, the gate voltage being sequentially output from the output terminals of the gate drivers; wherein the demultiplexer and the pixel are manufactured by the same manufacturing process.
According to the present invention, advantages similar to those of the above device can be obtained. Also, the number of gate drivers can be reduced, by providing the demultiplexer. Further, the demultiplexer and the pixels are manufactured by the same manufacturing process, so manufacturing can be carried out without increasing costs.
Further, the active matrix type liquid crystal display device according to the present invention comprises: a first shift register which sequentially shifts first start pulses and supplies output signals from each stage as gate voltage to one of the two gate lines provided on either side each of the pixel electrodes; and a second shift register which sequentially shifts second start pulses and supplies output signals from each stage as gate voltage to the other of the two gate lines provided on either side each of the pixel electrodes; wherein the first and second shift registers, and the pixel are manufactured by the same manufacturing process.
According to the present invention, advantages similar to those of the above device can be obtained. Also, providing the first and second shift registers does away with the need for external attachment of gate drivers. Further, the shift registers and the pixels are manufactured by the same manufacturing process, so manufacturing can be carried out without increasing costs.
Also, the active matrix type liquid crystal display device substrate according to the present invention comprises: a plurality of data lines and a plurality of gate lines provided on a substrate in a matrix form; and TFTs and pixel electrodes connecting to the TFTs provided on one side of each of the data lines, corresponding with each of the plurality of gate lines; wherein drain electrodes comprising the TFTs connecting to the pixel electrodes are provided on the same side as gate electrodes extending from the gate lines and comprising the TFTs, with a certain number of the data lines being electrically connected; and wherein the plurality of gate lines are provided so as to control the thin film transistors connected to each of the certain number of data lines, each by a different gate line.
Several types of configurations can be conceived for an active matrix type liquid crystal display device substrate whereby dots can be driven with fewer data lines than known arrangements, but the present invention further aims to provide a active matrix type liquid crystal display device with reduced flickering. This will be described next.
FIG. 24 is an example of a active matrix type liquid crystal display device substrate wherein the number of data lines has been reduced to half of that of known arrangements. This is an arrangement wherein dots from two columns PX (i, j) and PX (i, j+1) (wherein i=1 to m for both) are positioned on either side of a data line Dj+1 and both share the same data line Dj+1, whereby the number of data lines is reduced and the number of data drivers can be accordingly reduced. Also, in each row, the two dots which are on both sides of the data line Dj+1, e.g., dot PX (i, j) and PX (i, j+1) are driven by different gate lines GAi and GBi. As a result of such a configuration, the TFTs 121a and 121b for these two dots PX (i, j) and PX (i, j+1) are positioned in point symmetry with the center point of the two dots as the center thereof, and the positions of the drain and source of the TFTs 121a and 121b are reversed as to the gate (in the horizontal direction in the Figure).
FIG. 25A and FIG. 25B are diagrams illustrating the portion of the TFTs 121a and 121b of the above two dots PX (i, j) and PX (i, j+1). For the sake of simplicity in the description, the dot PX (i, j) to the left of the data line will be referred to as dot xe2x80x9caxe2x80x9d, and the dot PX (i, j+1) to the right of the data line will be referred to as dot xe2x80x9cbxe2x80x9d.
Generally, there is gate-drain parasitic capacity Cgd at the overlapping portion of the gate electrode and drain electrode of the TFT (in reality, the overlapping portion of the island and gate electrode also affects Cgd), but the area of this overlapping portion differs one from another due to the process precision (specifically, the alignment precision of the exposing apparatus) in the process of manufacturing, so there are irregularities in the gate-drain parasitic capacity Cgd.
In the event that the TFTS 121a and 121b of dot a and dot b are mutually positioned in point symmetry, and if the position of the drain layer to the gate layer is as designed as shown in FIG. 25A, the dimensions La and Lb of the overlapping portions of the gate electrodes 122a and 122b of the TFTs 121a and 121b of dot a and dot b and the drain electrodes 123a and 123b thereof are of equal dimension (including the dimensions from the island center to the drain electrode tip), and with the gate-drain parasitic capacity of dot a as Cgda, and the gate-drain parasitic capacity of dot b as Cgdb, Cgda=Cgdb holds. However, as shown in FIG. 25B, in the event that the drain layer shifts to the left direction in comparison with the gate layer for example, the dimensions Lxe2x80x2b of the overlapping portion of the gate electrode 122b and drain electrode 123b of the TFTs 121b of dot b becomes greater than the dimensions Lxe2x80x2a of the overlapping portion of the gate electrode 122a and drain electrode 123a of the TFTS 121a of dot a. Consequently, the relation in the gate-drain parasitic capacity of dot a and dot b is Cxe2x80x2gda less than Cxe2x80x2gdb (to be more precise, the overlapping portion of the island and gate electrode is also included in the parasitic capacity). That is to say, in the event that the TFTs are at positions of point symmetry in the substrate, the gate-drain parasitic capacity varies within the same substrate depending on the alignment precision of the exposing apparatus.
Now, in the event that gate voltage Vg is applied to a TFT, the field-through voltage xcex94Vp is expressed as follows:
xcex94Vp={(Cgd)/(Clc+Cs+Cgd)}xc2x7Vg 
wherein Clc represents liquid crystal capacity, and Cs represents storing capacity.
Accordingly, in the event that the gate-drain parasitic capacity Cgd differs, the field-through voltage xcex94Vp changes. Also, based on the relation between the field-through voltage and the offset voltage, change in the field-through voltage means that the offset voltage also changes, so differences in the gate-drain parasitic capacity changes the offset voltage. Accordingly, in the case of a TFT substrate of the above configuration, the offset voltage differs within the same substrate from dot to dot, so offset adjusting cannot be performed for all dots. This is what causes flickering.
The active matrix type liquid crystal display device substrate according to the present invention aims to maintain the idea of sharing a data line with a plurality of dots on the same row, while at the same time suppressing flickering owing to the process precision. To this end, as described above, the TFTs are provided on the same side of each of the data lines, and the drain electrodes of each of the TFTs are provided on the same side as the gate electrodes. That is, instead of positioning the TFTs in point symmetry, the positional relation of the source electrode and drain electrode of each TFT is the same for all of the TFTs on the substrate. Accordingly, even in the event that the alignment of the drain layer to the gate layer shifts, all of the TFTs on the substrate shift in the same direction, so the gate-drain parasitic capacity is equal for all TFTs, and consequently the offset voltage is uniform throughout the substrate. Hence, flickering can be suppressed.
Also, even in the event that data lines are provided for each column, the number of data lines can be reduced at the connection portion with the data driver by a certain number of data lines being electrically connected, so the number of data drivers can be reduced. Accordingly, the same data signals are supplied to the above certain number of data lines, but driving can be performed without obstruction by controlling each of the TFTs connected to each of the data lines of the electrically connected data lines with differing gate lines.
Also, it is preferable that the certain number of electrically connected data lines be mutually connected at both sides of the data lines, at least.
In the case of electrically connecting a certain number of data lines, mutual connection at only the one place on the side to be connected to the data driver is sufficient from a functional perspective, but an arrangement wherein the data lines are mutually connected at both sides can prevent line dropout, since even in the event that one place in a data line is broken, supply of the signals is not stopped. That is, this arrangement serves as a redundant structure regarding line dropout, thus improving yield.
Also, it is preferable that the configuration be an arrangement wherein the plurality of data lines are electrically connected at intervals of odd numbers, in increments of the certain number.
In order to improve display quality, and reduce cross-talk in particular, the so-called data line inversion driving method is generally used, wherein data signals of differing polarity are supplied to the even-numbered data lines and the odd-numbered data lines. However, with the present invention, in the event that neighboring data lines are electrically connected, dots corresponding with the electrically connected data lines may be of the same polarity, and the pixel potential be affected by coupling owing to parasitic capacity, so that data line inversion driving doers not effectively function.
Conversely, in the event that data lines are electrically connected at intervals of odd numbers, supplying signals of inverse polarity for each set of connected data lines ensures that the polarity will always be reverse between two neighboring data lines, so data line inversion driving effectively functions regarding an arbitrary dot. Consequently, cross-talk is reduced, and display quality is improved.
It is also preferable that the configuration be an arrangement wherein one or more but the certain number or less of gate lines are layered upon the pixel electrodes so as to traverse the pixel electrodes. In such an arrangement, the one or more but the certain number or less of gate lines, and the pixel electrodes cooperate to form storing capacity.
That is, with the present invention, the TFTS of each of the data lines of the electrically connected data lines are each controlled by different gate lines, so observation of one dot reveals a gate line for controlling another TFT traversing the dot. However, the area wherein the gate line traverses the dot can be used as storing capacity, so the arrangement wherein the gate lines traverse the dots does not lead to deterioration in aperture ratio.
Thus, the active matrix type liquid crystal display device according to the present invention can be constructed by holding liquid crystal between the above active matrix type liquid crystal display substrate and a substrate upon which a common electrode is provided.
With the active matrix type liquid crystal display device according to the present invention, the number of data drivers can be reduced as compared with known arrangements, which reduces the cost, and a liquid crystal device with high display quality can be provided, with reduced flickering.